X-ray tube

ABSTRACT

The invention relates to an X-ray tube which includes a device for at least substantially protecting an object to be examined against the incidence of undesirable X-rays (E) which can be produced notably by the decay of a residual or surplus charge present in a high-voltage circuit after an X-ray exposure. To this end there is provided at least one device ( 341, 342 ) for deflecting and/or defocusing the electron beam (E) produced by the residual and/or surplus charge in such a manner that at least it is not incident to a significant extent on a region ( 22 ) of an anode ( 2 ) wherefrom X-rays excited thereby are directed towards an object to be examined.

The invention relates to an X-ray tube which includes a device forprotecting an object to be examined at least substantially against theincidence of undesirable X-rays which can be produced notably by thedecay of residual or surplus charges still present in a high-voltagecircuit of the X-ray tube after the termination of an exposure.

EP 0 279 317 discloses an X-ray diagnostic device in which a thyristorcircuit is connected parallel to an X-ray tube. After the termination ofan X-ray exposure by interruption of the anode voltage (high voltage),the thyristor circuit is turned on by means of a drive circuit so that(residual) charges present in smoothing capacitances and cablecapacitances at the high-voltage side are eliminated mainly via thethyristor circuit and to a minor extent only via the X-ray tube. TheX-ray voltage and the X-ray current, and hence also the undesirableX-rays which are caused thereby and may still constitute a substantialpart of the imaging dose notably in the case of high tube voltages,short exposure times and small X-ray currents, even after the switchingoff of the high voltage, can thus be reduced faster and avoided,respectively, thus improving the edge steepness when the X-ray tube isswitched off.

It is a major drawback of this device, however, that almost the entireresidual charge to be eliminated has to be converted into loss heat inthe thyristor circuit. This has an effect notably in the case of fastpulsation so that the thyristor circuit should comprise components whichcan be loaded accordingly. Moreover, because of the comparatively large(notably parasitic) capacitances in the high-voltage circuit, the edgesteepness with which the tube voltage and the tube current can bereduced is not so great that an undesirable radiation dose can beavoided completely.

A similar problem is encountered in grid-controlled X-ray tubes in whichan exposure is not terminated by switching off the high voltage, but byblocking a grid. Granted, a tube current can no longer flow in thesetubes after the end of the exposure, so that no X-rays can be generatedeither. However, when a next exposure is to be carried out with a lowerhigh voltage, first the voltage difference which is due to (surplus)charges still present in the capacitances of the high voltage circuitmust be reduced in order to prevent the object to be examined from beingexposed to an undesirable radiation dose.

Finally, an undesirable radiation dose may also occur in X-ray tubescomprising two filaments, for example, when it is desired to switch overas quickly as possible from a fluoroscopy mode (with a small focus) toan exposure mode (with a large focus). Generally speaking, in X-raytubes of this kind only the filament intended for the small focus can beblocked by means of a grid, so that upon switching over to the otherfilament an electron beam can be generated, and hence X-rays excited,immediately when the filament current is increased, that is, alreadyduring the preparation phase for the exposure, as a result of (residual)charges (or possibly when the high voltage is switched on) still presentin the capacitances of the high-voltage circuit.

If, moreover, a preceding exposure was carried out with a higher highvoltage, it will first be necessary to reduce the voltage differencecaused by the (surplus) charges still stored in the capacitances of thehigh-voltage circuit before the lower high voltage is actually present,so that initially the object to be examined is again exposed to anundesirable radiation dose in this case.

The undesirable radiation dose as mentioned in the above cases isgenerally generated by soft X-rays which burden or can even damage inparticular the skin of a patient.

Therefore, it is an object of the invention to provide an X-ray tubesuch that an object to be examined can be protected at leastsubstantially against undesirable X-rays which could be produced inparticular by the decay of residual or surplus charges present in ahigh-voltage circuit after an X-ray exposure.

It is notably an object of the invention to provide an X-ray tube suchthat an object to be examined is not burdened by undesirable X-rayscaused by the decay of surplus charges stored in a high-voltage circuitduring the execution of an exposure with a high voltage (anode voltage,kV voltage) which is lower than that used during a preceding exposure.

It is also an object to provide an X-ray tube such that, even in thecase of fast switching over from the fluoroscopy mode to the exposuremode, notably by switching over between two filaments, no undesirableX-rays are generated during the preparation phase in which a filamentwhich is not to be blocked by means of a grid is heated due to residualcharges and/or surplus charges which are still present in thecapacitances of the high-voltage circuit and originate from a precedingexposure.

Finally, in the case of an X-ray tube without a grid it is also anobject to eliminate at least substantially undesirable X-rays which areproduced by residual charges in a high-voltage circuit after theswitching off of the high voltage and lead to inadequate edge steepnessof the X-rays after the switching off.

In conformity with claim 1 the object is achieved by means of an X-raytube which includes a first device which can be activated between twosuccessive exposures in order to generate a deflection pulse fordeflecting and/or defocusing an electron beam, generated by a residualor surplus charge present in a high-voltage circuit of the X-ray tube,in such a manner that at least it is not incident to a significantextent on a region of an anode wherefrom X-rays excited thereby aredirected towards an object to be examined.

In this context a “significant extent” is to be understood to mean anextent such that X-rays are generated which expose an object to beexamined to an undesirable radiation dose, that is, a radiation dosewhich cannot be used in a defined manner for imaging or which notablyburdens the skin of a patient.

A special advantage of this solution resides in the fact that it is notnecessary to switch currents at the high-voltage side and that thecharge is not converted into a loss power in electrical components, sothat no power circuits are required in this respect. What is more, aswitching unit in accordance with the invention is rather small andeconomical and can be realized so as to be integrated in the vicinity ofthe tube, for example, in the tube head.

Furthermore, the solution in accordance with the invention not onlyenables a reduction but also complete elimination to be achieved forundesirable X-rays as described above.

In addition to these advantages, the solution in accordance with theinvention also enables faster pulsed operation with an even greater edgesteepness for X-ray tubes which are switched on and off by the switchingof the high voltage.

The dependent claims relate to further advantageous embodiments of theinvention.

The embodiment disclosed in claim 2 is intended notably forgrid-controlled X-ray tubes for which claim 6 discloses a preferredembodiment.

The claims 3 to 5 relate to preferred embodiments of the first devicewhereby particularly simple and effective deflection and/or defocusingof the electron beam can be carried out.

The embodiment in conformity with claim 7 offers the advantage that thedeflected electron beam cannot reach the surroundings in an uncontrolledmanner.

The claims 8 and 9 relate to a preferred high-voltage generator for anX-ray tube, which generator is also suitable for operating the devicesin conformity with the claims 1 to 6.

Further details, features and advantages of the invention will becomeapparent from the following description of preferred embodiments whichis given with reference to the accompanying drawing. Therein:

FIG. 1 shows a circuit diagram of a first embodiment of the invention;

FIG. 2 shows a circuit diagram of a second embodiment of the invention;

FIG. 3 shows a circuit diagram of a third embodiment of the invention;

FIG. 4 shows a circuit diagram of a fourth embodiment of the invention;and

FIG. 5 shows a block diagram of a voltage supply unit for an X-ray tube.

Identical or corresponding parts or components in these Figures aredenoted by the same reference numerals.

The embodiments shown concern notably grid-switched or grid-controlledX-ray tubes in which an exposure is terminated by the switching orblocking of a grid (GCF: grid-controlled fluoroscopy; GAF: grid-assistedfluoroscopy etc.).

The principle in conformity with the invention can also beadvantageously used for the previously mentioned X-ray tubes with twofilaments (for example, for a small focus and a large focus) wheredirect switching over takes place from a grid-switched fluoroscopy modeto an exposure mode in that the filament (large focus) which is not tobe blocked by means of a grid and serves for an exposure mode is heatedto a high temperature.

Finally, the principle in accordance with the invention can also be usedfor the previously mentioned X-ray tubes without a grid where anexposure is terminated by switching off the high voltage and where theresidual charge still present in the capacitances of the high-voltagecircuit becomes manifest as an insufficient edge steepness with whichthe X-rays decay.

The embodiments to be described hereinafter thus serve for the at leastsubstantial protection of an object to be examined against the incidenceof undesirable X-rays which can be caused notably by the decay ofresidual or surplus charges still present after an X-ray exposure.

Such a residual or surplus charge is notably the charge which, after thetermination of an X-ray exposure, prevails in high-voltage leads,secondary windings of high-voltage transformers or other, notablyparasitic capacitances of the high-voltage circuit.

In order to protect an object to be examined, an electron beam generatedby this charge is deflected and/or defocused by means of a deflectionpulse in such a manner that it is not incident, or at least not to asignificant extent, on the part of the anode on which the X-raysintended for imaging are excited in normal operation of the X-ray tube.Such deflection can be realized by means of an electrical and/ormagnetic field; alternatively or additionally defocusing of the electronbeam can also be carried out by appropriate control of electron lensesor other devices, such as deflection coils, which serve to concentratethe electron beam in normal operation of the X-ray tube. In the case ofa grid-controlled X-ray tube, or a filament which is blocked by a grid,the electron beam is to be eliminated by switching through the tube bymeans of a discharge pulse.

FIG. 1 is a diagrammatic representation of a first embodiment of theinvention.

An X-ray tube in accordance with the invention thus includes essentiallya tube envelope 1 of glass or metal which encloses a vacuum space inwhich an anode 2 and a cathode 3 are accommodated. Between the anode 2and the cathode 3 there is situated an anode voltage source 4 wherebythe anode voltage (high voltage) is generated.

The anode 2 consists in a customary manner of an anode disc 21 with abeveled edge zone 22 as well as an anode rod 23. Also provided is astator 24 via which the anode 2 is rotated.

The cathode 3 includes a filament transformer with a primary winding 31and a secondary winding 32 for at least one filament 33, as well as acathode tube (Wehnelt cylinder) which is divided into two parts in thepresent embodiment so that it consists of a first tube half 341 and asecond tube half 342 which are electrically insulated from one another.There is also provided a bias voltage source 41 which is connected tothe first tube half 341 and can also be connected at option to thesecond tube half 342 by closing a switch 41 a, so that the first andsecond tube halves 341 and 342 are positively biased relative to thefilament 33.

Finally, a radiation collector 11 is provided on the wall of the tubeenvelope or on a protective tube housing (head), which collector is madeof, for example, lead or another material which at least substantiallyabsorbs an incident electron beam and may be formed by a suitablyreinforced wall zone of the protective tube housing or the tube envelope1.

In the exposure mode the X-ray tube generates an X-ray beam in acustomary manner by accelerating, while the switch 41 a is closed, theelectrons released from the filament 33 and focused by the cathode tube341, 342 by means of the anode voltage, said electrons being incident inthe form of an electron beam on the edge zone 22 of the anode disc 21;therefrom the X-rays thus excited are directed towards the object to beexamined via a window in the tube envelope 1.

After the termination of an exposure by the switching over of the biasvoltage source 41 in such a manner that the cathode tube 341, 342 isnegatively biased relative to the filament 33 so that the electron beamis interrupted, an occasionally very substantial residual charge remainsin the anode voltage circuit also after the switching off, ifapplicable, of the high voltage; such a residual charge must beeliminated prior to the beginning of an exposure, in particular if thisexposure is to be performed with a lower high voltage. When the highvoltage is not switched off but switched over directly to the lowervalue, the relevant surplus charge must be eliminated.

To this end, the switch 41 a is opened and a suitable positive voltageis generated by way of the bias voltage source 41, so that the firsttube half 341 is positively biased relative to the filament 33 and alsorelative to the second tube half 342. Thus, on the one hand the tube isswitched through so that an electron beam E is generated because of theresidual or surplus charge, and on the other hand the electron beam E isdeflected in the direction of the radiation collector 11 because of theunequal potentials on the two tube halves 341, 342. Said positivevoltage on the bias voltage source 41 thus represents a deflection pulseand a discharge pulse at the same time.

The residual or surplus charge is thus eliminated, without excitingX-rays on the anode 2.

In this respect it is to be noted that the deflection and dischargepulse is generated only when the switch 41 a is open. The period of timeduring which the switch is open and the duration of the discharge anddeflection pulse are proportioned in such a manner that the residual orsurplus charge is eliminated essentially completely. Subsequently, animaging exposure can be carried out with a defined high voltage andhence with a desired X-ray dose.

FIG. 2 shows a second embodiment of the invention, parts which aresimilar to or correspond to parts shown in FIG. 1 being denoted by thesame reference numerals.

In this embodiment the glass envelope 1 again accommodates an anode 2with an anode disc 21 with a beveled edge zone 22 and an anode rod 23which is driven via a stator 24. Also provided is a cathode 3 with atwo-piece cathode tube which comprises a first tube half 341 and asecond tube half 342 as well as a filament transformer with a primarywinding 31 and a secondary winding 32. The at least one filament 33,being connected to the secondary winding 32, can be biased positively ornegatively relative to the cathode tube 341, 342 by means of a firstbias voltage source 41.

There is also provided a second bias voltage source 42 whereby the firsttube half 341 can be biased positively or negatively relative to thefilament 33. Using a third bias voltage source 43, the second tube half342 can also be biased relative to the filament 33 by means of apositive or negative voltage by closing a switch 43 a.

Finally, the anode voltage source 4 is connected to the anode 2 and, viathe first bias voltage source 41, to the filament 33.

In order to eliminate the residual or surplus charges after an X-rayexposure, a bias voltage source 41 generates a discharge pulse wherebythe filament 33 is negatively biased relative to the cathode tube 341,342, so that the X-ray tube is switched through and the residual orsurplus charge produces an electron beam E.

This electron beam E is deflected at the same time to the radiationabsorber 11 by means of a deflection pulse because, while the switch 43a is open, a sufficiently high positive potential relative to thefilament 33 is applied to the first tube half 341 by means of the secondbias voltage source 42. Alternatively, in the closed position of theswitch 43 a and using appropriate anti-phase control of the second andthird bias voltage sources 42, 43, a positive potential relative to thefilament 33 can be applied to the first tube half 341 and a negativepotential relative to the filament 33 to the second tube half 342.

It is to be noted that the deflection pulse is not generated later thanthe discharge pulse, but simultaneously therewith or preferably somewhatearlier. The durations of these pulses are such that the residual orsurplus charge is essentially completely eliminated. A subsequent X-rayexposure can then be carried out with a defined high voltage and hencewith a desired X-ray dose.

FIG. 3 shows a third embodiment of the invention in which parts whichare similar or identical to those shown in FIG. 1 or FIG. 2 are againdenoted by the same reference numerals.

The X-ray tube shown in FIG. 3 again includes, accommodated in a tubeenvelope 1 of glass or metal, an anode 2 with an anode disc 21, abeveled edge zone 22 as well as an anode rod 23 which is rotated via astator 24. Furthermore, there is provided a cathode 3 with a primarywinding 31 and a secondary winding 32 of a filament transformer, as wellas a filament 33. As opposed to the embodiments shown in the FIGS. 1 and2, the cathode 3 in this case comprises a one-piece cathode tube 34.This cathode tube 34 can be positively or negatively biased relative tothe filament 33 by means of a first bias voltage source 41.

Furthermore, this third embodiment is provided with a first and a seconddeflection plate 51, 52 which are arranged so as to face one anotherbetween the anode 2 and the cathode 3. In order to bias the firstdeflection plate 51 positively or negatively relative to the cathodetube 34, a second bias voltage source 53 is connected between the twodeflection plates. Finally, a third bias voltage source 54 can beconnected, via a switch 54 a, to the second deflection plate 52 in orderto bias this plate positively or negatively relative to the filament 33.

In the exposure mode this X-ray tube generates an X-ray beam in knownmanner in that, in the closed state of the switch 54 a and while thefilament 33 is negatively biased relative to the cathode tube 34 bymeans of the first bias voltage source 41, the released electrons arefocused by the deflection plates 51, 52, accelerated by the anodevoltage and directed, in the form of an electron beam, towards the edgezone 22 of the anode disc 21 where they excite the imaging X-rays.

After the termination of an exposure, notably by switching over thefirst bias voltage source 41 in such a manner that the cathode tube 34is negatively biased relative to the filament 33 so that the tube isblocked, the residual or surplus charge must again be eliminated priorto the start of a new exposure with a lower high voltage.

To this end, the first bias voltage source 41 generates a dischargepulse whereby the filament 33 is negatively biased relative to thecathode tube 34, so that the X-ray tube is switched through and theresidual or surplus charge produces an electron beam E.

At the same time, or briefly before that, in the open state of theswitch 54 a the second bias voltage source 53 generates a voltage(deflection pulse) which biases the first deflection plate 51 positivelyrelative to the filament 33, so that the electron beam E is directedonto the radiation collector 11. Alternatively, in the closed state ofthe switch 54 a, such deflection of the electron beam E can also beachieved by making the second and the third bias voltage source 53, 54generate appropriate voltages of opposite phase whereby the firstdeflection plate 51 is biased positively relative to the filament 33whereas the second deflection plate 52 is biased negatively relative tothe filament 33.

The durations of these pulses are proportioned such that the residual orexcess charge is eliminated essentially completely. Subsequently, animaging exposure can again be performed with a defined high voltage andhence with a desired X-ray dose.

As stated before, the deflection plates 51, 52 serve to generate anelectric field whereby the electron beam E is directed towards theradiation collector 11 and/or whereby it is defocused. The deflectionplates 51, 52 can thus also be provided, if desired, on the externalwall of the tube envelope 1, so that the interior of the X-ray tube neednot be modified. On the other hand, the are also known X-ray tubes whichcomprise focusing plates or focusing electrodes for focusing theelectron beam on the edge zone 22 of the anode disc 21. Generallyspeaking, in such a case the focusing plates can also be used as thedeflection plates 51, 52 for deflecting the electron beam E towards theradiation collector 11.

Therefore, this embodiment is particularly advantageous for X-ray tubeswhich include deflection plates which are provided for the focusing ofthe electron beam in normal operation.

FIG. 4 shows a fourth embodiment of the invention in which parts whichare identical or similar to parts shown in the FIGS. 1 to 3 are againdenoted by the same reference numerals.

This embodiment again comprises, accommodated in a tube envelope 1, ananode 2 with an anode disc 21 with a beveled edge zone 22 as well as ananode rod 23 which is rotated via a stator 24. There is also provided acathode 3 with a one-piece cathode tube 34 as well as a filament 33which is fed with current from a filament transformer which includes aprimary winding 31 and a secondary winding 32.

Furthermore, between the cathode 3 and the anode disc 21 there isprovided a first deflection coil 61 which is connected to a firstcurrent source 63, as well as a second deflection coil 62 which is fedby a second current source 64. However, the deflection coils 61, 62 mayalso be situated outside the tube envelope 1, notably on the externalwall thereof. Furthermore, a quadrupole which is formed by a combinationof four coils can be used instead of the two deflection coils.

Finally, this embodiment also includes a bias voltage source 41 forpositively or negatively biasing the cathode tube 34 relative to thefilament 33 as well as an anode voltage source 4 for applying an anodevoltage between the anode 2 and the cathode 3.

After the termination of an X-ray exposure by blocking the tube by meansof the cathode tube 34, being negatively biased relative to the filament33 by the bias voltage source 41, the residual or surplus charge isagain eliminated prior to the beginning of a new exposure. To this end,the bias voltage source 41 generates a discharge pulse whereby thefilament 33 is negatively biased relative to the cathode sink 34.

At the same time there is generated a deflection pulse in that eitherthe first and/or the second current source 63, 64 activates the firstdeflection coil 61 or the second deflection coil 62, thus generating amagnetic field such that the electron beam E produced by the residual orsurplus charge is deflected towards the radiation collector 11 and/or isdefocused until this charge has been eliminated.

Subsequently, an imaging exposure can be carried out again with adefined high voltage, and hence with a desired X-ray dose.

This embodiment is particularly advantageous for X-ray tubes whichinclude magnet coils which are provided, for example, for focusing theelectron beam in the normal exposure mode.

The individual characteristics of the described embodiments can also becombined, if desired.

In all embodiments the electron beam E produced by the residual orexcess charge can also be directed towards the central part of the anodedisc 21, so that the radiation collector 11 can be dispensed with. It isimportant only that it is not incident on the edge zone 22 of the anodedisc 21 wherefrom the X-rays thus generated are reflected in thedirection of the object to be examined.

Alternatively or additionally to the deflection, the electron beam E canalso be defocused in all embodiments to such an extent that it isincident on the edge zone 22 with a low intensity only, so that theamount of X-rays thus generated is so small that it is acceptable. Tothis end, for example, focusing devices (electron lenses, coils or thelike), already present in the X-ray tube, can be suitably defocused bysuitable electrical control by means of a defocusing pulse.

Generally speaking, the radiation collector 11 is constructed as asuitably reinforced wall segment of the tube envelope 1. Alternatively,it may also be formed as a separate element absorbing the electron beam.

If necessary, additional elements can be provided for the absorption ofthe X-rays generated on the radiation collector 11 or the central partof the anode disc 21.

The discharge pulse and the deflection pulse can both be generated by asuitable circuit arrangement which includes either said first to thirdbias voltage sources or current sources 41; 42, 43; 53, 54; 63, 64, orwhich suitably controls voltage sources present in an X-ray system. Thecircuit arrangement is activated either automatically or by a user ofthe relevant X-ray system after an imaging X-ray exposure.

In the case of X-ray tubes comprising a stationary anode instead of ananode disc 21, the electron beam E produced by the residual or surpluscharge can also be directed towards an existing catching cage.

The solution in accordance with the invention can also be used in thecase of metal can tubes having a metal housing. It is then additionallypossible to direct the electron beam E, produced by the residual orsurplus charge, by way of a deflection pulse in the form of theinterruption of the anode voltage, towards the positive metal housing,thus preventing the excitation of undesirable X-rays.

In the case of the previously mentioned X-ray tubes without a grid, inwhich the X-rays are switched on and off by switching the high voltage,a deflection pulse of the kind set forth can be used to achieve asubstantial enhancement of the edge steepness with which the X-raysdecay after the termination of an exposure, that is, notably in the caseof pulse operation of the X-ray tube. Such a deflection pulse can beapplied simultaneously with or briefly before the switching off of thehigh voltage to suitable deflection plates (51, 52) or magnet coils (61,62) of such an X-ray tube, so that the electron beam produced by theresidual charge is deflected away from the anode and/or is suitablydefocused and the X-rays decay with a substantially greater edgesteepness.

Finally, the principle of the invention, according to which an electronbeam is deflected or defocused, can also be used in the case where thiselectron beam does not originate from a residual or surplus charge to beeliminated, but is produced in a different manner.

FIG. 5 is a diagrammatic representation of a circuit arrangement as wellas an X-ray tube 100 with an anode 2, two filaments 331, 332 of acathode as well as a cathode tube 34 for blocking and switching throughthe tube. The filaments 331, 332, one of which is provided for a largefocus (LF) while the other is provided for a small focus (SF), receive afilament current via a filament transformer (not shown). A high-voltagesupply unit, consisting of a first stage 110 and a second stage 111,forms an anode voltage (kV voltage) from an alternating voltage which issupplied by a converter (not shown). The Figure also showsdiagrammatically a voltage or current supply and control unit 120 whichcan be activated by an operator via a switch 122 which is connected bymeans of an optical conductor 121. The unit 120 is connected to thefirst stage 110 of the high voltage supply unit as well as, via aconverter 123, to the filaments 331, 332 and the cathode tube 34.

The unit 120 serves to generate the described discharge pulses wherebythe X-ray tube 100 is switched through, as well as to generate at thesame time the deflection pulses whereby the electron beam arisingbecause of the residual or surplus charges is deflected towards aradiation collector (not shown).

The voltage or current supply and control unit 120 may also beconstructed so as to form part of a high-voltage generator for an X-raytube.

1. An X-ray tube which includes a first device (42, 43, 341, 342; 51,52, 53, 54; 61, 62, 63, 64) which can be activated between twosuccessive exposures in order to generate a deflection pulse fordeflecting and/or defocusing an electron beam (E), generated by aresidual or surplus charge present in a high-voltage circuit of theX-ray tube, in such a manner that at least it is not incident to asignificant extent on a region (22) of an anode (2) wherefrom X-raysexcited thereby are directed towards an object to be examined.
 2. AnX-ray tube as claimed in claim 1, which includes a second device (41;42, 43) which can be activated between two successive exposures in orderto generate a discharge pulse for triggering the electron beam (E) sothat the residual or surplus charge present in the high-voltage circuitof the X-ray tube is eliminated at least substantially.
 3. An X-ray tubeas claimed in claim 1, in which the first device includes a dividedcathode tube with a first tube half (341) and a second tube half (342)of a cathode (3) of the X-ray tube, as well as at least one voltagesource (42, 43) whereby the tube halves (341, 342) can be connected todifferent electrical potentials in order to trigger the electron beam(E) and to deflect and/or defocus the triggered electron beam (E).
 4. AnX-ray tube as claimed in claim 1, in which the first device includes atleast one deflection plate (51, 52) which is connected to a voltagesource (53; 54) in order to generate an electrical field whereby theelectron beam (E) is deflected and/or defocused.
 5. An X-ray tube asclaimed in claim 1, in which the first device includes at least onedeflection coil (61, 62) which is connected to a current source (63; 64)in order to generate a magnetic field whereby the electron beam (E) isdeflected and/or defocused.
 6. An X-ray tube as claimed in claim 2, inwhich the second device includes at least one voltage source (41; 42,43) whereby the electron beam (E) is triggered by switching through theX-ray tube.
 7. An X-ray tube as claimed in claim 1, which includes aradiation collector (11) towards which the deflected electron beam (E)is directed.
 8. A voltage or current supply and control unit for anX-ray tube as claimed in claim 1 which serves to generate deflectionpulses for deflecting and/or defocusing an electron beam (E), producedby a residual or surplus charge present in a high-voltage circuit of theX-ray tube, in such a manner at least it is not incident to asignificant extent on a region (22) of an anode (2) wherefrom X-raysexcited thereby are directed towards an object to be examined.
 9. Avoltage or current supply and control unit for an X-ray tube as claimedin claim 2 which serves to generate discharge pulses for triggering theelectron beam (E) so that the residual or surplus charge present in thehigh-voltage circuit of the X-ray tube is eliminated at leastsubstantially.
 10. A high-voltage generator for an X-ray tube as claimedin claim 1 or 2, which includes a voltage or current supply and controlunit (120) as claimed in claim 8 or 9.